Electric incandescent projection lamp



Dec. 8, 1964 J. G. CARDWELL ETAL 3,160,776

ELECTRIC INCANDESCENT PROJECTION LAMP Filed D80. 1. 1960 H a m Q w Ovn%ww wk/i n ./A 3 m w b United States Patent 3,lttl,776 ELECTRIC INCANDESCENT PRUJECTIGN LAMP John G. Car-dwell, Cleveland Heights, and Man E. Arnold, Euclid, Ohio, to General Electric Company, a corporation of New York Filed Dec. l, 196%), Ser. No. 73,964 Claims. ll. 313 112) This invention relates to electric incandescent projection lamps, and more particularly to lamps embodying an internal reflector associated with the filament to project a light beam in a desired manner.

In such lamps it is desirable to employ a reflector coniprising a glass base member having a reflecting coating on a surface thereof. It is further desirable to arrange that the current-carrying lead-in wires extend through the reflector from the back to the coated front surface thereof. The filament is mounted on said lead-in Wires at the front of the reflector in accurate relationship thereto. The lead-in wires thus serve to support the reflecto-r, at least in part, and are thereby so located as to provide a minimum of interference. with the light reflected from the reflecting surface.

It has been noted in such lamps, particularly lamps having a filament of relatively high wattage dissipation in a relatively small bulb and a reflector substantially filling a cross section of the bulb, that there is a tendency to rather poor lumen maintenance, or depreciation of light output, during the life of the lamp. Accordingly, it is a principal object of the present invention to provide means to minimize such loss of light output.

For a full understanding of the invention attention is directed to the following detailed description and to the drawing wherein:

FIGS. 1 and 2 are side and front elevations of a lamp comprising the invention;

FIG. 3 is an end view, with the bulb in section, of the lamp disposed in a horizontal position;

Fit 4- is a fragmentary section of the assembly of the glass reflector and one of the lead-in wires;

FIG. 5 is a fragmentary section, otherwise corresponding to Fig. 3, of a lamp having a modified form of refiector, lead-in wire and filament assembly; and

PEG. 6 is a fragmentary elevation, from the back, of the reflector and leadin wire assembly of the FIG. 5 modification.

Referrins to H68. 1 and 2, the lamp illustrated therein by Way of example, comprises a sealed tubular glass bulb or envelope 1 containing a gaseous filling, for example nitrogen or argon at a pressure of about 1000 mm. at room temperature. The bulb contains a filament 2 herein illustrated as a coiled coil of tungsten wire extending transversely of the bulb and offset from the longitudinal axis thereof. The filament 2 is mounted at the focus of an internal reflector 3 of any desired shape, for example ellipsoidal, the filament being mounted at one of the foci of the ellipsoid. The filament 2 is connected across and between the ends of a pair of transverse inner lead-in wire portions 4 which extend through the reflector 3 and also serve as a support therefor.

The ends of the lead wire portions 4 at the back of the reflector 3 are connected to, and supported by, intermediate lead Wire portions 5 which'are, in. turn, connected to and supported by the ends of a pair of outer lead wire portions or pins 6 which extend through a discshaped pressed glass stern (not shown) which closes the end of the bulb l. The reflector 3 is additionally supported in any suitable manner, in this case by the hookshaped end 7 of the bight portion of a hairpin-shaped support wire '7 which is secured at its ends to the ends of a pair of dummy leads or pins 8 which also extend 3,160,776 Patented Dec. 8, 1964 ICC through the aforesaid stem at the end of the bulb 1. The lower end of the bulb 1 carries a base 9 and its upper end or dome is masked by an opaque coating 10 which may be an internal vapor-deposited coating of metal such as an aluminum-copper-iron alloy.

The reflector 3 is a pressed glass base member (of the usual lime or lead glasses used in lamps) having on its front surface a reflecting coating 11 preferably of vapor deposited material such as silver or a composite dichroic coating which reflects light but transmits the'heat or infrared rays incident thereon from the filament 2. Such dichroic coatings are Well known and may consist of an extremely thin coating of suitable metal or a composite coating constructed of a film of a semiconductor such as germanium, silicon, antimony sulfide or selenium, coated with a thin film or films of -a dielectric material or materials such as zinc sulfide, magnesium fluoride, aluminum oxide, magnesium oxide, etc. For instance, there may be employed a layer of germanium of such thickness as to transmit about 20% of visible radiations and coated with a layer of zinc sulfide having an optical thickness of about 250a. Other types of coatings are disclosed, for example, in patent 2,660,925 Turner wherein multiple coatings of pairs of dielectric films are used to increase the visible reflectance.

The manner of attachment of the reflector 3 to the lead-in wires is illustrated in FIG. 4 wherein the reflector is provided with a pair of hollow bosses 12 through which extend the respective lead wires 4 and to which the reflector is attached by glass beads 13 which are fused to the bosses l2 and around the lead wires 4.

As thus far described, the lamp operates satisfactorily when mounted in the vertical or base down position shown in FIGS. land 2. However, when operated in a. horizontal postion (as shown in FIG. 3) there is a considerable depreciation in light output during life of the lamp. Examination of the lamp disclosed the presence on the reflecting surface 11 of a thin and fairly uniform layer of black sooty material of the type generally referred to as tungsten blackening.

It will be evident that, in horizontal operation of the lamp as in FIG. 3, with the reflector 3 substantially filling the cross section of the bulb 1, the degree of gas stagnation is considerably greater than in vertical operation (as in FIG. 1) where there is room for the hot gases to pass up and around to the rear of the reflector. It was suspected that, during operation of the lamp, the glass of the reflector 3 is heated to a temperature at which it becomes sul'ficiently conductive to permit substantial electrical leakage.

Accordingly, it was theorized as follows. it is known that a hot tungsten filament emits copious quantities of electrons. In addition to the electrons, metal from the filament is vaporized into the gas stream where it condenses into a metallic fog or smoke of particles composed of many molecules. It is these tungsten smoke particles that deposit on internal parts of the lamp to form the familiar tungsten blackening. probably have some electrons attached to them, giving them a negative charge.

In the stagnant condition of horizontal burning, a space charge could develop in the gas and extend for some distance from the filament 2. These charges would tend to migrate outward under their own repelling forces toward any surface with a lower potential. If this surface were suificiently isolated from the filament circuit of the lamp, the charges reaching it would soon raise it to the same potential as the gas region itself and further charge drift would be repelled such that the rate of deposit of charged tungsten particles would be greatly reduced.

If, however, the surface has a conductive path to the filament circuit, then the charge would continue to drain These particles would areas/vs off and never build up a repelling potential. Tungsten particles would continue to flow to and deposit upon the surface. In the case of the silvered reflector surface ll, this deposit serves to reduce the reflectance and thereby lower the light output of the lamp. Although glass is generally considered to be a good insulator at normal temperatures, it becomes a much poorer insulator at elevated temperatures.

As a test of the foregoing theory, a negative voltage should be found to appear on the silver surface ill of the reflector 3 where the electrons and charged particles deposit. The particular design of the lamp permits readily measuring this voltage, since the reflective surface 11 is electrically connected to the dummy leads or pins 8 by the support wire '7'. This permits measurement of the electrical potential between the surface it and the filament circuit by connecting a vacuum tube volt meter or similar high impedance meter to the respective terminals.

With the lamp in a horizontal position, this potential was found to rise to a value of about one to three volts negative and, after a brief warm-up, to drop to a value of only 0.2 volt. Resistance measurements taken between the reflector 3 and the power leads 6 after warm-up showed a resistance of the glass reflector of about 2060 ohms.

Further checks were made with an otherwise similar lamp having a reflector like the glass reflector 3 but made of solid metal and insulated from the lead wires 4- by a ceramic insulator of aluminum oxide. in that case the rapid deterioration of light output did not occur. Measurements of the potential between the metal reflector and the filament circuit showed it to remain at about 0.8 to 1.5 volts. Resistance measurements between the metal reflector and the filament circuit through the ceramic insulator showed a value of about 150 to 200 megohms. As a further check, the metal reflector lamp was burned with an external leakage resistance of 200 ohms connected between the reflector and the filament circuit. In that case, the tungsten soot deposited on the reflector and reduced the light output.

The source of the difiiculty having been found, the solution became clear. A much higher resistance value, preferably in excess of about 100 megohrns, was needed between the power leads and the surface of the glass reflector which attains a temperature of about 400-450 C during operation of the lamp.

One solution, compatible with existing design features of the lamp, and equipment used in the manufacture thereof, is to provide both the lead-in wires 4 with a coating 14- (Fig. 4) of material having the required high resistance. The coating may be applied as a slurry of suitable refractory oxide into which the leads are dipped.

Satisfactory insulation has been obtained with a coating 14 of alumina ceramic slurry composed essentially of aluminum oxide in a binder of nitrocellulose and amyl acetate. After dipping the leads 4 into the slurry, they are dried and then preferably coated with a lacquer to protect the ceramic coating from being scraped off during insertion of the leads into the reflector 3. The protective coating is preferably composed of an acrylic lacquer such as a solution of one or both methyl methacrylate and isobutyl methacrylate in acetone and methyl ethyl ketone, It is burned off during sealing of the leads into the reflector so that it is not present in the finished lamp. A coating of alumina about one and a half mils thick gives to the combination of glass reflector and coated leads a resistance of approximately 1000 megohms meas ured in an oven at 300 C.

Tests have shown that lamps made with insulated leads maintain an average of about 90% of their initial light output after eight hours of burning as compared to about maintenance for similar lamps without such insulation.

It will be evident that similar advantages may be oball tained by alternative techniques such as flame spraying of ceramic materials, use of ceramic oxides other than alumina, or other materials such as anodized aluminum coated steel leads.

A presently preferred coating, which combines good insulation with good adherence, is formed from an insulating slurry comprising aluminum oxide, aluminum hydroxide and phosphoric acid. The slurry may be prepared by first forming a powder mixture in proportions of 1060 grams aluminum oxide, 60 grams aluminum hydroxide and 609 cc. water which is ball milled for one hour, after which 300 cc. additional water is mixed in, the mixture is poured into a shallow pan, dried in an oven at 110 C., and broken up into a powder. A liquid is then formed by adding 224 cc. of phosphoric acid to 700 cc. water. A slurry is then formed by ball milling a mixture of 300 grams of the aforesaid powder mixture and 206 grams of the said liquid. This slurry should not be kept more than two days since it reacts at room temperature.

To apply the coating, the preheated leads 4 are dipped into the slurry to form a uniform coating of about 0.682 inch thick, if the slurry thickens, it may be thinned by mixing in additional water. The leads are then heated slowly in an oven to drive off moisture without causing bubbles. The coating is then baked by heating with direct open fires to a temperature of about 300 C. whereby the rcactizon of the ingredients is completed to form a hard coating. The coated leads are then assembled with the reflector 3 (FIG. 4) by inserting them through the hollow bosses l2, dropping a glass bead over each lead 4 and fusing the bead to the boss 12 and to the coating 14 on the lead 4.

in the next step, the silver coatin 11 is vaporized onto the glass reflector in vacuo. Then the assembly of reflector 3 and leads 4 is placed in a fixture or jig and the filament 2 is welded to the leads d to be accurately located thereon with respect to the focus of the reflector. The assembly of reflector 3, leads 4 and filament 2 is then placed in a jig with the assembly of lead wire portions 5, support wire 7' and pins 6 and 8 which are carried by the flat glass stem press (not shown), after which the pairs of lead wire portions and 5 are welded together. The resulting assembly is assembled with the glass bulb 1 which is sealed to the said stem press, after which the bulb l is evacuated, filled with gas, and sealed off at the usual exhaust tube (not shown) depending from the said stern press.

By virtue of the above construction, it becomes possible to utilize a coated glass reflector and to support the reflector by the power leads 4% which extend through the reector at the apex thereof to thereby provide a support for the reflector and minimize interference of the leads with the projected beam. The glass reflector has certain cost advantages as compared to a solid metal reflector. Even more importantly, it permits the use of a dichroic filter coating to minimize projection of heat radiation in the light beam.

In the modified structure shown in FIGS. 5 and 6, the power leads 4' are electrically insulated or isolated from the glass reflector 3 by a plug member 15 of insulating material which may be a ceramic composition such as aluminum oxide. The generally rectangular plug 15 has an enlarged inner endlo which is held against the inner surface of the reflector at the periphery of a corresponding rectangular slot therein, by a spring member 17 in the form of a hairpin-shaped tungsten wire located in a longi tudinal slot 18 in the plug 1 5. The leads 4 extend through longitudinal bores 19 in the plug 15 and are thereby insulated by the plug 15 from the glass reflector 3.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. An electric incandescent projection lamp comprising a sealed gasfilled glass envelope, a glass base reflector memher in said envelope having a reflecting coating on a front surface thereof, said reflector member substantially filling a cross section of said envelope and thereby impeding gas How in the envelope around said reflector member, a pair of lead-in conductors extending into said envelope and through said reflector member to the front thereof, an incandescent filament connected across said lead-in wires at the front of said reflector member, said filament dissipating sufficient wattage to heat the glass base of said reflector member to a temperature at which it conducts an appreciable current flow, and insulating means interposed between said lead-in wires and said glass base reflector member, said insulating means having a resistance materially higher than that of said glass base and sufficient to prevent current flow through said glass base to said lead-in Wires.

2. A lamp as set forth in claim 1 wherein said insulating means has a resistance exceeding about 100 megohms.

3. An electric incandescent projection vlamp comprising a sealed gas-filled glass envelope, a glass base reflector member in said envelope having a reflecting coating on a front surface thereof, said reflector member substantially filling a cross section of said envelope and thereby impeding gas flow in the envelope around said reflector member, a pair of lead-in conductors extending into said envelope and through apertures in said reflector member to the front thereof, said reflector member being secured to said lead-in wires by giass seals, on incandescible filamerit connected across said lead-in Wires at the front of said reflector member, said filament dissipating sufiicient wattage to heat the glass base of said reflector member to a temperature at which it conducts an appreciable current how, and insulating means interposed between said lead-in wires and the glass sealing said wires to said glass base reflector member, said insulating means having a resistance materially higher than that of said glass base and sufficient to prevent current flow through said glass base to said iead-in wires.

4. A lamp as set forth in claim 3 wherein said insulating means is a refractory ceramic coating on said lead-in wires comprising aluminum oxide.

5. A lamp as set forth in claim 1 wherein said reflecting coating is a dichroic filter coating which is highly reflective of light rays and highly transmissive of infrared rays emitted by said filament.

References @ited in the file of this patent UNET ED STATES PATENTS 2,614,524 Haynes Oct. 21, 1952 2,980,818 Harris et al Apr. 18, 1961 FOREIGN PATENTS 803,536 Great Britain Oct. 29, 1958 833,761 Great Britain Apr. 27, 1960 

1. AN ELECTRIC INCANDESCENT PROJECTION LAMP COMPRISING A SEALED GAS-FILLED GLASS ENVELOPE, A GLASS BASE REFLECTOR MEMBER IN SAID ENVELOPE HAVING A REFLECTING COATING ON A FRONT SURFACE THEREOF, SAID REFLECTOR MEMBER SUBSTANTIALLY FILLING A CROSS SECTION OF SAID ENVELOPE AND THEREBY IMPEDING GAS FLOW IN THE ENVELOPE AROUND SAID REFLECTOR MEMBER, A PAIR OF LEAD-IN CONDUCTORS EXTENDING INTO SAID ENVELOPE AND THROUGH SAID REFLECTOR MEMBER TO THE FRONT THEREOF, AN INCANDESCENT FILAMENT CONNECTED ACROSS SAID LEAD-IN WIRES AT THE FRONT OF SAID REFLECTOR MEMBER, SAID FILAMENT DISSIPATING SUFFICIENT WATTAGE TO HEAT THE GLASS BASE OF SAID REFLECTOR MEMBER TO A TEMPERATURE AT WHICH IT CONDUCTS AN APPRECIABLE CURRENT FLOW, AND INSULATING MEANS INTERPOSED BETWEEN SAID LEAD-IN WIRES AND SAID GLASS BASE REFLECTOR MEMBER, SAID INSULATING MEANS HAVING A RESISTANCE MATERIALLY HIGHER THAN THAT OF SAID GLASS BASE AND SUFFICIENT TO PREVENT CURRENT FLOW THROUGH SAID GLASS BASE TO SAID LEAD-IN WIRES. 